Onshore and offshore hydrocarbon development increasingly targets deep reservoirs. During drilling, a variety of pressure, temperature, and pore-filling fluid regimes can be encountered. Kick and loss-of-well control events during drilling represent risks to personnel, infrastructure, the environment, and the economics of the well. Maintaining well control in the borehole requires appropriate use of over-balanced and underbalanced drilling techniques. Maintaining the proper balance between mud weight—the primary tool for controlling this balance between the borehole and the surrounding formation and pore-filling fluids—is a significant challenge.
The invention of while-drilling technologies and sensors have been instrumental in the measurement, monitoring, characterization, and communication of borehole, near-borehole, and far-borehole environments in real-time. The principal while-drilling technologies are referred to as: Logging-While-Drilling (LWD) and Measurement-While-Drilling (MWD). LWD is generally used to describe instrumentation for the measurement of formation rock and fluid properties, such as electrical resistivity and bulk density. MWD is used to describe instrumentation that provides data related to drilling mechanics, wellbore deviation, directional surveys, and data transmission to the surface in real-time. In both MWD and LWD, the tools are incorporated into the drill string and are located in drill collars near the drill bit. These technologies provide feedback as the well is drilled, and have effected better decision making for engineers during the drilling process in order to improve safety, reduce costs, and enhance efficiency.
Most drilling activities for oil and gas are carried out using over-balanced drilling where the drilling fluid's pressure is maintained above the formation's pore pressure. Despite industry best practices and the use of sophisticated technology, the drilling fluid's pressure may fall below the formation pressure and result in the influx of formation fluid into the wellbore during drilling or a “kick”. Basic methods of kick detection currently in-use include mud logging, where drilling operations rely on mud returns to the rig floor to identify when a well being drilled is taking on a kick of liquid hydrocarbons, gas, or water from the surrounding subsurface formation. Acoustic methods have also been utilized, which involve generating pressure waves in the drilling mud, and monitoring travel times to determine if the drilling mud is diluted by dissolved gas (a/k/a “gas-cut”).
In some conventional LWD/MWD tools (e.g. gamma or neutron density, acoustic velocity, electrode or ring electrical resistivity, and electromagnetic induction), a source transmits a measurement medium (e.g. energy or neutrons) into the wellbore and surrounding geologic formation. The measurement medium interacts with the material in the standoff (e.g. drilling fluid and drill cuttings); materials which can transmit or attenuate the measurement medium. Measurement medium attenuation occurs by deflecting the measurement medium backward to the source (a/k/a back-scatter). The back-scatter because of standoff attenuation is detected by the instrumentation sensors, and signals from multiple sensors are analyzed in order to provide a compensated measurement reflecting a property of the geologic formation. The compensation involves the use of multiple detectors at different spacings from the instrumentation source, and is achieved because the lateral depth-of-investigation by geophysical instrumentation into a geologic formation is primarily dependent on the source-receiver spacing. As the source-receiver spacing increases, the subsurface volume comprised of both the standoff and the geologic formation that is investigated by the geophysical instrumentation also increases. The increase in investigated volume means the geophysical instrumentation has a greater lateral depth-of-investigation into the geologic formation. Once the investigated borehole volume extends beyond the wellbore standoff and into the geologic formation, it is then possible to account for the standoff effect within the formation measurement, which allows isolation of the geologic formation measurements.
To date, LWD and MWD data has focused on evaluation and analysis of the geologic formation and pore-filling fluids in the near-wellbore environment. Information related to the standoff fluids has typically been treated like noise and filtered out. This disclosure focuses on the standoff background data (a/k/a “noise”) for use as early indicators of changes in the composition and physical properties of the borehole-filling fluids (e.g. the drilling mud, formation fluids, and associated formation cuttings) that are indicative of an influx of water, gas or liquid hydrocarbons from the surrounding geologic formation. Early kick detection can be obtained by using the principals of wellbore geophysics with advanced modeling techniques, and existing borehole geophysical data to monitoring real-time changes in the physical properties (electrical resistivity or conductivity, compressional (p-wave) velocity, and bulk density) of the drilling mud in the standoff near the bit.
These and other objects, aspects, and advantages of the present disclosure will become better understood with reference to the accompanying description and claims.